Form the start- I liked the difficulty! For me it hit that spot, where it's difficult enough to be satisfying , but not too much, so you don't get frustrated and ragequit :)
The boss changing weapons was a nice touch! I really thought I was done when he pulled the rocket launcher (but turns out the shotgun was harder).
The gameplay reminded me of Hotline Miami, which is always a good thing!
The graphics... could be better. Personally I work with a graphics guy, because I'm bad at it myself :)